If you've ever stood on a beach and watched the tide go out, you might have wondered where all that water actually goes.
I do this all the time so I decided to research the subject an put my mind at rest.
One minute the sea can be right up against the shoreline, and a few hours later, around 6 to be precise, the beach can stretch much further out towards the horizon. It can look as though the ocean has simply disappeared.
But where has ALL that water gone?
Of course, the water hasn't actually gone anywhere quite as dramatic as vanishing. It's still there – just somewhere else.
It's all about the movement of the oceans
The main reason we get tides is the gravitational pull of the moon, with the sun also playing an important part in this. As the Earth rotates and the moon orbits around it, huge amounts of seawater are constantly being pulled around the World's oceans.
When the tide is coming in, water is moving towards the coastline. When the tide is going out, that water is moving away from the coast and into deeper water.
So when you're standing on a beach at low tide, the sea hasn't disappeared. The water has effectively been redistributed across the surrounding ocean.
Why does the tide seem to go so far out?
This can be particularly noticeable around the UK. Some parts of the coastline have a much greater tidal range than others, meaning the difference between high water and low water can be several metres.
The shape of the coastline makes a big difference too. Wide, shallow beaches and estuaries can make the falling tide particularly obvious. As the water moves away, areas that were underwater only a few hours earlier can be exposed, revealing rocks, sand, mud and rock pools.
In places such as the Bristol Channel, the tidal range can be especially impressive, which is why the coastline can look completely different depending on the time of day.
Where does the water go at low tide?
There isn't a giant empty space in the ocean waiting for the tide to return. The water is simply moving as part of its continuous cycle.
As water levels fall along one section of coastline, water is being pushed and drawn towards other areas. The oceans are enormous, and the movement of tidal water takes place across huge distances.
There can also be local differences caused by the shape and depth of the seabed, islands, headlands, bays and estuaries. These features can change both the timing and height of the tide from one location to another.
Why does the sea come back?
After low tide, the process reverses and the water level begins to rise again. This is the incoming tide that we're all familiar with.
If you've ever sat on a beach for a few hours, you can actually see this happening. Rocks become covered, pools disappear beneath the water and eventually the shoreline moves back towards where it was at high tide.
It can sometimes happen surprisingly quickly, which is why it's important to keep an eye on the tide when walking across beaches, exploring tidal areas or visiting places where you can become cut off from the shore.
The sea never really went anywhere
So next time you're standing on a beach and wondering where the sea has gone, the simple answer is that it's still there. The water has moved away from that particular part of the coastline as part of the enormous tidal movement taking place around our planet.
And when you see the tide coming back in, you're watching that same process happen in reverse.
It's one of those everyday things that we see happening all the time, but when you stop and think about just how much water is moving, the tides are pretty incredible.
Have you ever visited a beach in the UK where the tide seems to go out an incredible distance? We'd love to hear about it in the comments below. Tell us where it was, how far the sea appeared to go out, or which beach you think has the most impressive tidal change.
Thank you very much to Carl Ablett for the photo of low tide at Broadmarsh in Hampshire
